1. Field of the Invention
The present invention relates to a transmission power controlling circuit for use with a wireless communication system such as a cordless telephone system or a wireless LAN of which each base station is not operated, in particular, in accordance with a transmission power controlling circuit for use with a system corresponding to code division multiple access method.
2. Description of the Related Art
In such a conventional power controlling circuit disclosed in Japanese Patent Laid-Open Publication No. 8-149563, as a terminal unit of a cordless telephone system moves away from a main unit thereof, the power controlling circuit of the terminal unit increases the power level of a transmission signal. In contrast, as the terminal unit approaches the main unit, the power controlling circuit of the terminal unit decreases the power level of the transmission signal. Thus the quality of a transmission signal can be always maintained at a constant level.
FIGS. 1A and 1B are block diagrams showing the structures of the main unit and the terminal unit of the cordless telephone system disclosed in Japanese Patent Laid-Open Publication No. 8-149563, respectively.
A transmitting/receiving portion 1 of a radio portion of the main unit shown in FIG. 1A modulates transmission data into a narrow band, band-spreads the resultant data with a spread sequence, and generates a transmission signal. The transmission/reception portion 1 also de-spreads a received signal with a spread sequence, obtains a narrow band signal, and demodulates the resultant signal. A transmission power amplifier 2 and a reception amplifier 3 amplify the transmission/reception signals. A transmission/reception signal sharing unit 4 is used to share an antenna 5 with the transmitted signal and the received signal. A channel table 7 stores spread sequence numbers of an up link signal transmitted from the terminal unit to the main unit and a down link signal transmitted from the main unit to the terminal unit in the order of frequency in use. A reception quality measuring portion 8 measures the received signal and the power level of a received noise. A controlling portion 6 controls the amplification factor of the transmission power amplifier 2 so that the quality of the transmission signal is always maintained in a constant level.
FIG. 1B shows the structure of the terminal unit of the cordless telephone system. The difference between the structure of the main unit and the structure of the terminal unit is in that the terminal unit does not have the channel table 7. In the main unit and the terminal unit, the reception quality level obtained by the reception quality measuring portion 8 (15) is supplied to the transmitting/receiving portion 1 (9) through the controlling portion 6 (14). The transmitting/receiving portion 1 (9) places the reception quality level to the transmission signal.
In the system shown in FIGS. 1A and 1B, even if the terminal unit is away from the main unit, the quality of the transmission signal can be always maintained at constant level. On the other hand, when the terminal unit approaches the main unit, since the quality of the transmission signed is maintained at a constant level, the power levels of the transmission signals of both the main unit and the terminal unit decrease. Thus, the power consumption of the terminal unit decreases and thereby it operates for a long time. In addition, since the power level of the transmission signal decreases, the interference of the transmission signal to another system decreases.
However, in the prior art reference, when the terminal unit is away from the main unit, a signal is transmitted at a high power level.
In other words, when the terminal unit transmits a signal at a large power level near a main unit of another system, the signal interferes with the other system and thereby the line capacity thereof decreases. Sometimes, the signal causes the other system not to operate.
An object of the present invention is to provide a transmission power controlling method for use with a spread spectrum communication system that shares the same frequency with other systems and operates independently without interfering with the other systems.
The present invention is a spread spectrum Communication system having a plurality of mobile stations and a plurality of base stations that share the same frequency and operate independently, wherein each of the base stations may be identified as a local base station or as a non-local base station relative to each respective mobile station and wherein each base station has a means for always transmitting a pilot signal (base station reference signal) at a constant power level, and wherein each of the mobile stations has a quality measuring means for measuring base station signals received from the local base station and an adjacent base station identified as one of the non-local base stations relative to a relevant mobile station, and a transmission power controlling means for controlling the power level of a signal transmitted from the relevant mobile station corresponding to the measured results in such a manner that when the relevant mobile station approaches the adjacent base station, the transmission power controlling means decreases the power level of the transmission signal.
According to the present invention, the mobile station measures the power level of the transmission signal of the local base station and the power level of the transmission signal of an adjacent base station, compares the power levels, and controls the power levels corresponding to the compared results. For example, when the mobile station determines that the mobile station is close to the local base station, as the mobile station moves away from the local base station, the mobile station increases the power level of the transmission signal. In contrast, as the mobile station approaches an adjacent base station, the mobile station decreases the power level of the transmission signal. Thus, the interference to the adjacent base station decreases. Consequently, the communication efficiency of the entire radio communication system improves.
These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of a best mode embodiment thereof, as illustrated in the accompanying drawings.